popping & crackling noise from restored valve AM radio - diyAudio
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Old 9th May 2012, 04:20 PM   #1
budkor is offline budkor  United Kingdom
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Default popping & crackling noise from restored valve AM radio

Hi All

Last year, I restored an old valve radio from 1950s. It was not working when I got it, but when I recapped all the original paper capacitors to new ones, it started working.

It is AC Mains powered, and sounds quite good - and it is still working well. But when switched on, and then after about 4 hours constant receiving AM stations, it starts sounding popping and crackling from the reception.

What could this be? Where should I look to sort this problem? Thanks for your info and advice.

Last edited by budkor; 9th May 2012 at 04:24 PM.
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Old 9th May 2012, 04:58 PM   #2
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by budkor View Post
Hi All

Last year, I restored an old valve radio from 1950s. It was not working when I got it, but when I recapped all the original paper capacitors to new ones, it started working.

It is AC Mains powered, and sounds quite good - and it is still working well. But when switched on, and then after about 4 hours constant receiving AM stations, it starts sounding popping and crackling from the reception.

What could this be? Where should I look to sort this problem? Thanks for your info and advice.
It could be a lot of things..

The first easy one is, is it still their with the volume turned all the way down..If it is then chance is either PSU caps or leakage in the output stage possible coupling cap. If its gone when the volume is down then its in the reciever stage..again possible coupling cap..

Might be easier with a schematic..

Sorry its a bit vague...

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M. Gregg
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Old 9th May 2012, 05:04 PM   #3
banat is offline banat  Serbia
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It is usually start crackling and popping only in late afternoon and evening time ?, but never in the morning time ?
Did you replace main rectifier elko ?
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Old 9th May 2012, 05:13 PM   #4
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Check for a false contact in a tube, or in a tube socket. Try to strike the tubes with a short piece of plastic or wood, then listen the sound it makes. Also, check if it is not an external interference.

Luck!!!
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Old 9th May 2012, 05:29 PM   #5
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I once had an old Zenith radio which developed similar crackling noises.
I traced the problem to the IF transformers. Inside the transformer cans, Zenith had designed fixed capacitors. The capacitors were made up of a pair of flat metal plates with a thin sheet of mica between the plates. The mica had deteriorated and the two metal plates had started to arc through the mica.
I was able to replace the mica with a thin sheet of teflon.
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Old 9th May 2012, 05:41 PM   #6
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the two metal plates had started to arc through the mica.
Arcing? There are few RF volts across a IF coil winding. Surely, they make false contact and then introduced noise, but arcing?? Mmmmm....
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Old 9th May 2012, 05:52 PM   #7
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Yes ... arcing. Zenith designed the radio so that the IF tube plate current went through the transformer primary. Strange design but it did work for many years.
Attached Images
File Type: jpg Zenith IF.jpg (79.8 KB, 76 views)
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Old 9th May 2012, 05:53 PM   #8
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Arcing? There are few RF volts across a IF coil winding. Surely, they make false contact and then introduced noise, but arcing?? Mmmmm....
Yes but lots of DC!

I would tap the 35W4 (35Z5 in older versions) tube as that is most likely to be the problem. Next try the 50C5 (50L6)

Also look to see if any of the tubes are glowing red on the plates.

FB we cross posted!
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Old 9th May 2012, 06:43 PM   #9
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No, no and no. The circuits gives no DC voltage across primary winding, as the logic and common sense tell me. So, no voltage = no arc.
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Old 9th May 2012, 06:49 PM   #10
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No, no and no. The circuits gives no DC voltage across primary winding, as the logic and common sense tell me. So, no voltage = no arc.
The original arc may have been from the primary to the grounded can.
The mica showed a few holes.
Whatever ... it did arc through the mica. As you can see on the schematic, I marked "REP" to indicate that I had replaced the mica insulator.
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